An on-vehicle device is often coupled to various devices (referred to as USB devices, hereinafter) via USBs (Universal Serial Buses). The on-vehicle devices include a car audio, a car navigation device, etc., and the USB devices include a hard disk (HD) drive, a music player, a sensor, etc.
For example, the on-vehicle device has a USB control unit (a USB control apparatus). The USB control unit is coupled to a USB connector. The on-vehicle device is coupled to the USB device by coupling a connector of the on-vehicle device to the USB connector of the USB device via a USB cable. The USB cable includes a power supply line, a communication line and a ground cable.
The USB control unit is coupled to the power supply line for supplying the USB device with electrical power. The USB control unit supplies electrical power with a voltage of +5 V (a maximum current capacity of 0.5 A) for the USB device coupled to the on-vehicle device via the USB connector and the power supply line.
The USB device operates based on electrical power supplied from the USB control unit. A guaranteed working voltage range of the USB device complies with specifications. In general, it is DC+5V±5%. The specifications include USB2.0.
JP2008-305148 A discloses related art.
If the USB cable is used to connect between the USB control unit and the USB device, there is a voltage drop in the USB cable. If the length of the USB cable is long, the power supply voltage applied to the USB device via the power supply cable may become lower than a permissible value specified by the specifications because of the voltage drop. The USB device may be unable to operate because the power supply voltage applied to the USB device drops. In other words, because of power consumed by a line resistance which is proportional to the length of the USB cable, the power supply voltage from the power supply of the USB control unit drops and no longer meets the guaranteed working voltage range DC+5V±5% of the USB device.
The maximum transmission distance between the USB control unit and the USB device is specified by the specifications. For example, according to USB2.0, the maximum transmission distance is 5 m. The maximum transmission distance corresponds to the length of the USB cable connecting between the USB control unit and the USB device. Thus, it is necessary to dispose the USB control unit and the USB device such that the length of the USB cable doesn't exceed the distance.
However, depending on a circumstance in a vehicle on which the length of the USB cable is installed, there may be a case where the USB control unit cannot be disposed near the USB device. It can be anticipated that the distance between the USB control unit and the USB device exceeds the maximum transmission distance. If the distance between the USB control unit and the USB device exceeds the maximum transmission distance, a signal transmitted by the signal cable is attenuated to have amplitude below a permissible value specified by the specifications.
In some cases, if the distance between the USB control unit and the USB device exceeds the maximum transmission distance, an amplifier called as a repeater or a booster is provided between the USB control unit and the USB device. The attenuation of the signal is compensated for by the repeater and the voltage drop is compensated for by the booster. However, there may be a case where it is difficult to provide the repeater and/or the booster.